Question

(i) How will you bring about the following conversions :

(I) Bromobenzene to 1-phenylethanol

(II) Benzene to m-nitroacetophenone

(ii) An organic compound with the molecular formula \( \mathrm{C_8H_8O} \) forms 2,4-DNP derivative, reduces Tollens' reagent and undergoes Cannizzaro reaction. On vigorous oxidation with acidic or alkaline \( \mathrm{KMnO_4} \) it gives Benzene-1,2-dicarboxylic acid. Identify the compound and write the products when it undergoes Cannizzaro reaction.

(iii) Arrange the following compounds in increasing order of their acid strength :

\( \mathrm{C_6H_5COOH,\ O_2N - CH_2 - COOH,\ CF_3 - COOH,\ HCOOH} \)

Hint:

Use a Grignard reagent plus ethanal for the first conversion. Acylate first and nitrate second for the next. The compound is 2-methylbenzaldehyde. Stronger electron withdrawal gives a stronger acid.

Answer 1

Step 1: Understanding the Question:
The question has three parts. Part (i) asks for two conversions. Part (ii) asks us to identify a compound from its reactions. Part (iii) asks for an acid strength order.

Step 2: Part (i)(I), Bromobenzene to 1-phenylethanol:
1-Phenylethanol is a secondary alcohol. It is made by adding a Grignard reagent to an aldehyde.
First prepare phenylmagnesium bromide from bromobenzene and magnesium in dry ether.
\[ \mathrm{C_6H_5Br + Mg \xrightarrow{dry\ ether} C_6H_5MgBr} \]
Then add ethanal. The Grignard reagent attacks the carbonyl carbon. Hydrolysis of the adduct gives the alcohol.
\[ \mathrm{C_6H_5MgBr + CH_3CHO \rightarrow C_6H_5CH(CH_3)OMgBr \xrightarrow{H_3O^+} C_6H_5CH(OH)CH_3} \]

Step 3: Part (i)(II), Benzene to m-nitroacetophenone:
The acetyl group is meta directing. So the acetyl group must go in first, and nitration comes second.
Friedel-Crafts acylation of benzene with ethanoyl chloride and anhydrous \( \mathrm{AlCl_3} \) gives acetophenone.
\[ \mathrm{C_6H_6 + CH_3COCl \xrightarrow{anhyd.\ AlCl_3} C_6H_5COCH_3 + HCl} \]
Nitration with a mixture of conc. \( \mathrm{HNO_3} \) and conc. \( \mathrm{H_2SO_4} \) then puts the nitro group at the meta position.
\[ \mathrm{C_6H_5COCH_3 \xrightarrow{conc.\ HNO_3 / conc.\ H_2SO_4} m\text{-}O_2NC_6H_4COCH_3} \]
The order cannot be reversed, because nitrobenzene does not undergo Friedel-Crafts acylation.

Step 4: Part (ii), Identify the compound:
The formula is \( \mathrm{C_8H_8O} \). It forms a 2,4-DNP derivative, so it has a carbonyl group.
It reduces Tollens reagent, so it is an aldehyde.
It gives the Cannizzaro reaction, so the aldehyde group has no alpha hydrogen, which means \( \mathrm{-CHO} \) is attached directly to the benzene ring.
Vigorous oxidation gives benzene-1,2-dicarboxylic acid (phthalic acid). So the two side groups sit ortho to each other.
The remaining carbon is a methyl group. The compound is 2-methylbenzaldehyde (o-tolualdehyde), \( \mathrm{o\text{-}CH_3C_6H_4CHO} \).
In the Cannizzaro reaction, two molecules react with conc. NaOH. One is oxidised and one is reduced.
\[ \mathrm{2\ o\text{-}CH_3C_6H_4CHO + NaOH \rightarrow o\text{-}CH_3C_6H_4CH_2OH + o\text{-}CH_3C_6H_4COONa} \]
The products are 2-methylbenzyl alcohol and sodium 2-methylbenzoate. Acidification of the salt gives 2-methylbenzoic acid.

Step 5: Part (iii), Acid strength order:
Electron withdrawing groups stabilise the carboxylate ion and increase acid strength.
The phenyl group in \( \mathrm{C_6H_5COOH} \) is weakly electron donating by resonance with the carboxyl group, so benzoic acid is the weakest (pKa 4.2).
\( \mathrm{HCOOH} \) has no donating group (pKa 3.75), so it is stronger than benzoic acid.
The \( \mathrm{-NO_2} \) group in \( \mathrm{O_2N-CH_2-COOH} \) has a strong \( \mathrm{-I} \) effect (pKa 1.68).
Three fluorine atoms in \( \mathrm{CF_3COOH} \) give the strongest \( \mathrm{-I} \) effect (pKa about 0.2), so it is the strongest acid.

Final Answer:
(i)(I) \( \mathrm{C_6H_5Br \xrightarrow{Mg,\ dry\ ether} C_6H_5MgBr \xrightarrow{CH_3CHO;\ H_3O^+} C_6H_5CH(OH)CH_3} \).
(i)(II) Benzene, acylation with \( \mathrm{CH_3COCl/AlCl_3} \), then nitration with \( \mathrm{HNO_3/H_2SO_4} \).
(ii) The compound is 2-methylbenzaldehyde. The Cannizzaro products are 2-methylbenzyl alcohol and sodium 2-methylbenzoate.
(iii) Acid strength increases as shown below.
\[ \boxed{\mathrm{C_6H_5COOH \lt HCOOH \lt O_2N-CH_2-COOH \lt CF_3-COOH}} \]